1. Field of the Invention
This invention relates to switching power supplies in general and, more particularly, to switching power supplies utilizing inductive energy storage.
2. Description of the Prior Art
Typical switching power supplies have energy storage elements which, during one part of the switching cycle, have loaded therein energy which is later transferred to a load during a subsequent part of the cycle. Feedback is usually provided to regulate the output current or voltage from the supply to the load. Typical storage elements are inductors, such as a transformer. For purposes here, the switching cycle has two parts, the loading of the energy into the storage elements, called an "inhale" and the transfer of the energy from the storage element to the load, called an "exhale". If less than all the energy stored in the storage elements is transferred to the load, the remaining energy is dissipated prior to the beginning of the next inhale step. Most power supply designs go to great lengths to minimize the amount of excess energy. However, even the most efficient designs have excess energy remaining in the storage elements after an exhale. As will be discussed in more detail below, the dissipation of the remaining excess energy may cause excessive heating of components in the power supply as well as instability in the operation of the supply.
Although the elements and operation of a switching power supply are well known, a brief description is given herein. The typical switching power supply has a switching transistor, driven by a control circuit between cutoff and saturation, which loads the storage elements with energy from a power source. Controlling the amount of energy loaded into the storage element is determined by the length of time the transistor is conducting and the amount of current allowed into the storage elements thereby. This energy is approximately: ##STR1## where L is the combined equivalent inductance of the storage elements, i(t) is the current into the storage elements as a function of time, and T is the time the switching transistor is saturated (conducting). It is understood that this inductance may vary as a function of the current. Typically, the current function is an exponential; no shaping thereof is provided beyond the nominal current inrush into an inductor. For purposes here, only the amount of time the transistor is conducting controls the energy loaded during the inhale; the frequency of the switching cycle and the power source voltage are assumed to be fixed.
The control circuits determine the amount of time the switching transistor is conducting by feeding back the output current and/or voltage presented to the load. This feedback regulates the voltage and/or current going to the load to a predetermined amount. In addition, the power from the supply may be limited when a given current is drawn therefrom. For example, the switching power supplies shown in U.S. Pat. Nos. 4,499,532 and 4,704,670, assigned to the same assignee as this invention, provide a constant output power for use in subscriber line circuits. Here, the feedback, in addition to that discussed above, includes turning off the switching transistor when the current in the storage elements during the inhale exceeds a predetermined amount.
The dissipation of excess energy typically occurs in the storage elements (due to non-ideal inductors therein) and in the switching transistor. Transients caused by the switching transistor conducting prior to the full dissipation of the excess energy in the storage elements causes the excess energy to be dissipated therein. The transients are also introduced into the feedback circuits, particularly those which monitor the current flowing in the storage elements. These transients are then coupled back to the control circuits, which may lead to the upset of the control circuits, causing instability in the switching power supply.
Snubbers are one way of absorbing some of the excess energy while protecting the switching transistor from the transients. An exemplary snubber (205) is shown in FIG. 2 of the above-cited U.S. Pat. No. 4,499,532. The snubber 205, disposed across switching transistor S3, is a passive circuit relying on the diode D3 to bypass the resistor R20 when the transient has the desired polarity. However, it may absorb energy not considered to be in excess, making the switching power supply less efficient than desired. The energy dissipated by the snubber may exceed that which may be safely dissipated by an integrated circuit, making a monolithic switching power supply (except for the energy storage elements) unattainable.